By causing spacing out of the population the optimal use of the environment is achieved and inefficient individuals tend to be weeded out of the population. In many species the territoriality is only evident during the breeding season and this can be an important natural selection device to ensure that only fit individuals able to hold on to a territory will be able to breed and thus pass on their genes. Releaser stimuli are common in territorial behaviour patterns, the robin and the stickleback both respond to a red coloured area, which induces aggressive behaviour in territory defending males. Since these aggressive displays of behaviour are not really designed to result in too much physical damage there tend to be terminating stimuli for the aggressive behaviour.